Numerous applications require controlling a defined environment's air quality characteristics, specifically providing an environment having a narrow range of relative humidity or preventing the creation of extreme relative humidity conditions, i.e. less than 10% or greater than 90% RH. As an example, when shipping environment-sensitive cargo such as optical devices, electronics, drugs, organisms, human organic material, seeds, electronics, mechanical devices, catalysts, chemicals, plants, art, or military devices, the shipping container which houses the cargo must maintain a range of relative humidity that best suits the type of cargo being shipped. Environment-sensitive cargo often require an environment having a defined range of relative humidity, i.e. greater than 50% or between -10% and 10%, to insure that the cargo will be preserved and maintained.
For example, the performance of carbon monoxide (CO) sensing systems is improved with the inclusion of a relative humidity control system. While carbon monoxide has no smell, visible appearance, or taste, it is very toxic. Because of energy conservation measures such as increased insulation and weather stripping, many buildings are tightly sealed, further leading to an increased likelihood of carbon monoxide poisoning.
Carbon monoxide detectors are examples of sensors that are designed to detect toxin levels accurately without costing over $100. Carbon monoxide sensors are often battery operated and made of a supramolecular chemical sensor/reagent coated onto a semi-transparent substrate. The reagent/substrate combination forms a solid state sensor system that responds to increasing exposure of carbon monoxide (CO) by means of a photochromic response, i.e., a change in electromagnetic radiation (e.g. light) reflected from the surface or a change in electromagnetic radiation intensity transmitted through the sensing element(s). Examples of such sensors are described in U.S. Pat. Nos. 5,063,164, 5,618,496, and 5,618,493. Additionally, reagents which can be used to detect CO are described in U.S. Pat. No. 4,043,934.
The materials used in CO detection are very sensitive to the extremes of relative humidity. For example, palladium and molybdenum salts, useful for carbon monoxide detection as described in Analytical Chemistry, Vol. 19, No. 2, pages 77-81 {1974), are only practical for use over short period of time because their accuracy is greatly affected by relative humidity.
While this salt technology was improved upon by adding a third metallic salt component which produces a self-regenerating catalyst, this method is short-lived. This catalyst, disclosed in U.S. Pat. No. 4,043,934, uses the impregnation of a carbon monoxide-sensitive chemical catalyst solution into powdered silica-gel substrates to give detectors sensitivity to low concentrations of atmospheric carbon monoxide. While this system is effective in detecting carbon monoxide, it has not met with commercial acceptance due to the short functional life of the catalyst and its inability to operate effectively at very low or high relative humidity, i.e. 5% or 95% RH, as compared to the more moderate range of relative humidity, i.e. 15% to 90% RH. Tests have shown that, at 100 parts per million (ppm), the reagents described in U.S. Pat. No. 4,043,934 do not properly function at less than 15% relative humidity or at above 90% relative humidity, if the material is in that environment for a long period of time, e.g., 168 hours or more.
Also, tests have shown that the carbon monoxide sensors described in U.S. Pat. No. 5,063,164 have adequate life but not enough sensitivity to satisfy the Underwriter Laboratory (UL) requirements listed in UL 2034, effective Oct. 1, 1998. The UL 2034 standards require continued sensor sensitivity after 168 hours of exposure to an environment having 15% relative humidity at 22.degree. C. and to an environment having 95% relative humidity at 52.degree. C. In other words, the sensing system should have sufficient sensitivity to detect 70 ppm of CO in an environment having 95% (.+-.4%) RH at 52.degree. C. for a period lasting between 1 hours and 4 hours, after having been first conditioned for 168 hours in an environment having 10% RH at room temperature.
The sensitivity of conventional CO sensors to relative humidity is of concern because, for a CO sensor system to be commercially useful, it must have a functional life of at least one year, preferably 5 to 10 years, and must operate over the full range of relative humidity found in the environment, i.e., 0 to 100% RH. These same concerns exist for the full range of biomimetic sensors described in U.S. Pat. Nos. 5,063,164, and 5,618,493 and for certain catalysts which operate effectively only within specific relative humidity and temperature ranges. For example, hopcolite works only under very dry condition and a Cu/Pd/Mo catalyst works well between 20 to 80% relative humidity.
Controlling the air quality, such as the relative humidity, of defined environments is especially critical to the proper functioning of military equipment. If the operative environments of military equipment have a high relative humidity, they will experience condensation on all cold surfaces whenever the equipment is carried up to a high altitude and returned rapidly to a low altitude. The operation of environment-sensitive equipment, such as computers, sensors, or weapon systems, can be easily compromised by this moisture, i.e. condensation, which has entered into the operative environments of the environment-sensitive equipment.
Additionally, corrosive or toxic warfare agents can impair the operation of military equipment or destroy the accuracy of these systems. For example, optical surfaces for infrared or other applications are particularly sensitive to corrosion and, therefore, susceptible to blurring or fogging. Furthermore, whenever combustion products or other battlefield material are emitted, they can cause a rapid corrosion of the military equipment. Therefore, air quality regulation of the storage and operative environments of military equipment is important to extend the life, effectiveness, and continued operation of the equipment.
Methods and devices for controlling relative humidity in a completely closed system have been known for many years. For closed systems that have limited movement and limited orientation, a typical ASTM procedure uses solid phase saturated salts to maintain specified relative humidity at a specified temperature. For example, magnesium nitrate can be used to maintain a relative humidity of 55% (.+-.5%) within a temperature range of 20 to 30.degree. C.; magnesium chloride can be used to maintain a relative humidity of 33% (.+-.3%) within a temperature range of 10 to 60.degree. C.; and sodium nitrite can be used to maintain a relative humidity of 66% (.+-.3%) within a temperature range of 20 to 30.degree. C. The CRC Handbook of Chemistry and Physics, 69th Edition, page E42 discloses tables of salts which will control the relative humidity in a closed system in a range of about 9% to 99% RH. Additionally, Stokes and Robinson, Ind. Eng. Chem. 41: 2013 (1949), discloses the use of various concentrations of sulfuric acid, sodium hydroxide, and calcium chloride to maintain a substantially constant relative humidity at 25.degree. C. Finally, to protect a device from high relative humidity, a dessiccant such as silica gel, calcium carbonate or similar water absorbing material, can be added before shipping.
These current methods of regulating relative humidity have, however, substantial disadvantages. Conventional methods and devices are successful at regulating the relative humidity of a defined environment when the surrounding real world environment has a relative humidity in the range of about 15 to 89% RH. In the real world, however, real world environments reach the relative humidity extremes of 100% RH and near zero RH, i.e. 4% RH. Therefore, there is a need for an improved device and method that can regulate relative humidity in a desired range despite the extreme relative humidity conditions of the surrounding environment.
Additionally, in some air shipping applications, it is desirable to have packaging containers that can release pressure upon reaching high altitudes, therefore requiring the package to have an opening to atmosphere. To control the relative humidity within such a pressure-release container, reservoir systems having a high surface to volume ratio and containing one or more salts, either as a solid or partially solid in solution, and water is desired. However, these relative humidity control systems are either completely closed, and therefore not suitable to a pressure-release container, or limited in their orientation relative to a chosen reference point, i.e. ground level.
Similarly, while an ASTM procedure on how to best control relative humidity in a closed system does exist, a method and/or device is needed to extend that relative humidity control to open systems which interact with the atmosphere, i.e. sensor systems that detect a gas in the air within a predetermined time at a predetermined concentration.
Finally, the use of dessicant material is also accompanied by significant limitations. Under prolonged exposure to high relative humidity, dessicant materials become saturated, become unable to absorb moisture, and, therefore, fail to prevent high relative humidity from damaging the environment-sensitive cargo. Although the labor to replace these desiccant materials is expensive, failure to do will result in damage to the environment-sensitive cargo.
In sum, while the control of air borne contaminants and relative humidity is critical to the preservation of innumerable types of materials and devices, there currently does not exist an air quality control system that is orientation independent, small, rugged, inexpensive, power independent, able to handle extremes in relative humidity, able to withstand long periods of dormancy without maintenance, and capable of lasting for extended periods of time.